The invention relates to a process that can be used for producing an organic anhydride, an organic acid halide, or combinations thereof.
Acid anhydrides and acid chlorides are important intermediates for the chemical industry.
Aliphatic and aromatic acid anhydrides have a variety of chemical uses, including the preparation of certain esters, such as cellulose esters and aspirin; the manufacture of alkyd resins; modification of the curing of certain epoxy resins and of polyester properties; as a retarder in rubber vulcanization; and in various syntheses, such as those of phenolphthalein, polypeptides, and peroxycarboxylic acids.
Acid chlorides, such as isophthaloyl and terephthaloyl chlorides are important polymer intermediates, for instance in the production of polymers for fibers and polycarbonates. Such applications require very low levels of impurities and by-products. Monofunctional contaminants can seriously reduce the molecular weight of a polyester and adversely affect the physical properties of fibers spun from the polymer. Traces of colored impurities can be unacceptable in polycarbonate products intended for optical uses.
Isophthaloyl chloride can be prepared from isophthalic acid by many routes. Commercially, the preferred chlorinating agent is phosgene, as described by Carnahan in U.S. Pat. No. 2,657,233. The uncatalyzed or xe2x80x9cthermalxe2x80x9d reaction is, however, slow. A problem is that, as the temperature is raised, the solubility of phosgene in the liquid phase decreases, limiting the acceleration of the reaction.
Consequently, numerous catalytic systems have been proposed for the reaction of acids with phosgene to produce the acid chlorides. A number of nitrogen containing compounds, e.g., amides and ureas are in use. An important example is dimethylformamide (hereinafter DMF) as described by Seagraves in U.S. Pat. No. 5,113,016. The active catalyst in the DMF process is the reaction product of DMF and phosgene. This material is thermally unstable, decomposing rapidly above 100xc2x0 C., especially in the absence of excess acid.
Thus, the MF/phosgene complex is only marginally stable under the reaction conditions, and impurities such as formylbenzoyl chloride and dichlorotoluoyl chloride are formed. Seagraves, in U.S. Pat. No. 5,113,016, cited above, reduced the amount of these impurities by introducing an oxidizing air sparge into the process. Other nitrogen-containing catalysts also have the disadvantage of marginal stability under the reaction conditions. Essentially all the impurities in the product using the DMF catalyst result from decomposition of the DMF catalyst. Japanese Patent Kokai: Sho 56-103134, Nagata, et al discloses a purification procedure involving the addition of certain metals and their salts, including titanium and titanium chloride, to the reaction product formed by the reaction of organic carboxylic acids and phosgene using lower aliphatic amides as the catalyst.
Decomposition of the DMF catalyst results in emissions of methyl chloride. Methyl chloride is of environmental concern as a xe2x80x9cgreenhouse gasxe2x80x9d. It also results in the formation of dimethylcarbamoyl chloride, a carcinogen, requiring responsible disposal, and careful control in the product. DMF decomposition further results in a tar purge stream requiring appropriate disposal and causing a yield loss. Finally, the DMF catalyst species is very corrosive, increasing the cost of materials of construction.
Other processes have been proposed using phosphorus compounds, nitrogen heterocyclic compounds, amines, and other catalysts. All possess some combination of disadvantages, high cost, higher reaction temperatures, lower yields, etc.
It would be desirable to develop a process for using organic acids to form the anhydride and, in the presence of phosgene, to form the acid chloride. The present invention provides such a process.
A process that can be used to produce an organic anhydride, an organic acid halide, or mixtures thereof is provided. The process comprises contacting a reaction medium with a catalyst in which the reaction medium comprises (1) at least one organic acid, (2) combination of the organic acid and phosgene, (3) combination of at least one organic anhydride and phosgene, or (4) combination of the organic acid, the organic anhydride, and phosgene; and the catalyst is a Group IVB transition metal halide.
The contacting of a reaction medium with a catalyst can be carried out under any suitable conditions. Generally, the reaction medium comprising at least one organic acid, mixtures of at least one organic acid and phosgene, or mixtures of at least one organic anhydride and phosgene can be contacted with the catalyst at an elevated temperature under dry conditions and optionally with a solvent. The catalyst is a Group IVb transition metal halide.
The contacting of at least one organic acid with the catalyst produces an acid anhydride or acid anhydrides. The contacting of organic acid with phosgene produces an acid chloride or acid chlorides. An organic acid chloride or acid chlorides can also be produced by contacting at least one organic acid anhydride and phosgene. No aliphatic amide is used in the process of this invention, avoiding the multiple problems of decomposition and corrosivity characterizing the prior art.
The organic acid can be a branched, straight chain, or cyclic alkanoic, alkenoic, or alkynoic acid; or an aryl, alkyl aryl organic acid having one or more carboxylic acids groups; or a mixture of such acids. Examples of suitable organic acids include, but are not limited to, aliphatic carboxylic acids of 2 to about 20 carbon atoms, and aromatic and cycloaliphatic carboxylic acids of 7 to about 24 carbon atoms, per molecule. The suitable acids can contain 1 to 3 carboxyl groups. For example, suitable aliphatic acids include, but are not limited to, acetic acid, butyric acid, lauric acid, palmitic acid, neo-pentanoic acid, propanoic acid, chloroacetic acid, dichloroacetic acid, succinic acid, adipic acid, sebacic acid, acrylic acid, methacrylic acid, succinic acid, and mixtures of two or more thereof; suitable aromatic acids include, but are not limited to, benzoic acid, m-nitrobenzoic acid, isophthalic acid, phthalic, phenylacetic acid, p-chlorobenzoic acid, trans-cinnamic acid, m-toluic acid, terephthalic acid, and mixtures of two or more thereof; suitable cycloaliphatic acid includes, but is not limited to, cyclohexane carboxylic acid; and suitable mixtures of acids are benzoic and terephthalic acids, and isophthalic and terephthalic acids.
The suitable acid anhydrides for the purpose of this invention are anhydrides of the general formulae: 
wherein each monovalent R or divalent Rxe2x80x2 independently represents an organic radical such as a hydrocarbon group. Particularly suitable acid anhydrides are those having an aliphatic, cycloaliphatic, or aromatic group. Thus, R or Rxe2x80x2 can be alkylene, alkenylene, cycloalkylene, arylene or like divalent, saturated and unsaturated radicals. Preferably, the number of carbon atoms in the R or Rxe2x80x2 groups is from 1 to 24 and more preferably 1 to 12 per group.
The organic acid anhydride can be any anhydride of the above-listed organic acids. Illustrative acid anhydrides include, among others, acetic anhydride, butyric anhydride, hexanoic anhydride, benzoic anhydride, trimellitic anhydride, octanoic anhydride, chloroacetic anhydride, acrylic anhydride, phenylacetic anhydride, adipic anhydride, sebacic anhydride, nitrobenzoic anhydride, chlorobenzoic anhydride, toluic anhydride, isophthalic anhydride, terephthalic anhydride, succinic anhydride, and mixtures of two or more thereof.
For the purposes of describing this invention, but not to limit the scope of the invention, isophthalic acid and other specific acids and anhydrides are used as example starting materials.
The catalyst is a Group IVb transition metal halide, such as titanium or zirconium tetrachloride or tetrabromide. The term xe2x80x9chalidexe2x80x9d as used herein to describe the Group IVb transition metal salt refers to chloride or bromide, preferably tetrachloride or tetrabromide. Titanium tetrachloride is the preferred catalyst. These Group IVb metal halides catalyze the conversion of acids to acid anhydrides, and, in the presence of phosgene, the conversion of acid anhydrides to acid chlorides. The novelty and effectiveness of this process stem from the combination of high catalytic activity and the thermal stability of the system. The process of this invention can result in the production of very high purity acid chlorides, the opportunity to recycle catalyst, and environmental and process advantages.
The optional solvent is chosen for inertness in the process. When an acid chloride is the product, the acid chloride is preferably substantially solubile in the solvent. The same solvents can also be used for producing an anhydride. Examples of suitable solvents include, but are not limited to, o-dichlorobenzene, chlorobenzene, heptane, and mixtures of two or more thereof. The acid chloride product can also be used as the solvent, for instance, in the preparation of isophthaloyl chloride from isophthalic acid. Solvents such as chlorobenzene and heptane can be removed from the product stream by fractional distillation. Where the acid chloride is the solvent, no solvent separation is necessary. The acid chloride can be separated by vacuum distillation. In cases where the volatility of the acid chloride is too low for vacuum distillation, other procedures well known to those skilled in the art can be substituted, such as filtration or centrifugation. Additionally the option of driving the conversion to the acid chloride to completion can be available. It should be noted that driving this reaction to completion is typically not practical using the nitrogen-based catalysts of the prior art, such as DMF, due to the tendency for the catalyst-phosgene complex to decompose excessively as reaction conditions are made more severe.
It is important to note that titanium tetrahalide does not react with phosgene as does DMF to form the active species. Rather it catalyzes the conversion of acid to anhydride, wishing not to be bound by theory, most likely by reaction at the isophthalic acid slurry surface to bring into solution a titanium trihalide salt. The mode of action of the Group IVb metal halide catalyst is completely different from that of DMF.
While not wishing to be bound by theory, it is believed that the reaction sequences for the catalyzed reactions of the present invention are described by Reaction Sequence A below, depicting reactions of isophthalic acid. Isophthalic acid, slurried in isophthaloyl chloride, is believed to react at the slurry surface with the TiCl4 catalyst to form the TiCl3 salt (I). The isophthaloyl chloride is effectively the solvent in this case. While isophthalic acid is relatively insoluble in the acid chloride (a property that would limit reaction rates), the TiCl3 salt is readily soluble in the isophthaloyl chloride, passing rapidly into solution where the salt reacts with isophthaloyl chloride to form the anhydrides II and III. In the isophthalic case, the anhydrides are external. Hydrogen chloride, but not carbon dioxide, is liberated in both the salt and anhydride formation steps. In contrast with the acid, the anhydrides are soluble in isophthaloyl chloride. In the presence of phosgene, the anhydrides react stepwise to form first the anhydride/chloride IV and finally two isophthaloyl chloride molecules. In the final two steps, carbon dioxide, but not hydrogen chloride, is liberated.
Reaction Sequence A 
In another embodiment of the invention, terephthalic acid can be contacted with titanium tetrahalide and phosgene to produce terephthaloyl chloride rapidly. Interestingly, if the phosgene is withheld and using titanium tetrachloride, the reaction mass of terephthalic acid and titanium tetrachloride rapidly solidifies as the bicyclic anhydride, in contrast to the isophthalic acid reaction, has a low solubility in the acid and a higher melting point. Reaction of a mixture of terephthalic acid and benzoyl chloride (e.g., a 50/50 molar mixture) with TiCl4 and phosgene avoids the solidification and the mixed acid chlorides are formed. The terephthaloyl chloride can be readily isolated by distillation and the benzoyl chloride recycled. Where a mixture of isophthalic chloride and terephthaloyl chloride is the desired product, isophthalic acid and terephthalic acid (e.g., a 50/50 molar mixture) can be contacted with TiCl4 and phosgene, solidification is prevented, and the reaction proceeds to produce mixed anhydrides, or mixed acid chlorides if phosgene is introduced.
The catalyst can be present in the reaction medium in an effective catalytic quantity. Generally, the amount of TiCl4 or ZrCl4 catalyst used can be about 0.001 to 0.1 moles of catalyst per mole of acid or anhydride, and preferably 0.005-0.05 moles of catalyst per mole of acid or anhydride. The reaction pressure is between 10 mm Hg to 20 atmospheres (130 Pa-200 kPa), and preferably ambient to 10 atmospheres (10 KPa-100 kPa). Increased pressure increases reaction rates by increasing the solubility of phosgene in the reaction mass, and lower or higher pressures can aid in stripping volatile products from the reaction mass or retaining product in the reaction mass. The freezing point of the solvent or reaction mass and the stability limits of the reactants, solvent, and products determine the temperature. Lower temperatures also increase the solubility of phosgene in the reaction mass. Typically, the temperature can be in the range xe2x88x9220xc2x0 C. to 250xc2x0 C. and preferably 50xc2x0 C. to 200xc2x0 C.
Phosgene can be introduced to the subsurface of the reaction medium with vigorous agitation to maximize gas liquid contact. Vented hydrogen chloride and unreacted phosgene can be conveniently scrubbed using excess 20% aqueous sodium hydroxide solution.
The advantages of the process of this invention include: 1. There are no species involved in the TiCl4 catalysis that provide facile pathways for thermal instability, 2. The evolution of hydrogen chloride in the formation of the anhydrides II and III prior to the introduction of phosgene, and carbon dioxide after the introduction of phosgene, occur separately, 3. The reaction mix of the present invention is substantially less corrosive than the DMF-catalyzed reaction mass, and 4. The Group IVb metal halide catalyst can be readily recovered and reused.
Advantage 1 results in substantially purer acid chloride than is obtained from the DMF-catalyzed process of the prior art. As discussed above, monofunctional impurities can adversely affect the molecular weight of polymers made from acid chloride monomers. Similarly, even traces of color in the acid chloride adversely affect optical grade polymers.
Advantage 2 improves the efficiency with which the evolved hydrogen chloride off-gas may be scrubbed and recycled, since the hydrogen chloride stream is not diluted with a large volume of carbon dioxide.
Advantage 3 allows less expensive materials of plant construction.
Advantage 4 minimizes catalyst cost and eliminates disposal costs for non-recyclable nitrogen-containing catalysts such as DMF.